1. Field of the Invention
This invention relates to a sound absorbing article and, more particularly, to a glazing e.g. a windshield, side window and/or rear window of a vehicle, having vibration dampening or sound absorbing properties and method of making same.
2. Discussion of the Presently Available Technology
As can be appreciated by those skilled in the art, vibration can have damaging effects on products that cause performance loss and failure, and transmit noise resulting in human discomfort.
Materials are available that may be used to reduce vibrations. One product is a family of viscoelastic polymers sold by 3M Corporation under the marks IDS 110, ISD 112, ISD 113 and SJ 2015. These polymers are advertised for use in dampening vibrations in, among other things, buildings, cars, electronic equipment, photography equipment, and air and space crafts.
Of interest in the present discussion is dampening vibrations that are produced by aerodynamic flow and acoustic excitation in glazings e.g. vehicle glazings such as windshields, side windows and/or rear windows of land, sea, air and space vehicles. In the categories of automotive glazings or transparencies, side and rear windows may be monolithic tempered glass sheets, double glazed windows having sheets of tempered glass, or laminated glass sheets that may be annealed glass sheets or heat strengthened sheets, and windshields may be annealed glass sheets laminated together by a plastic interlayer and sheets of monolithic annealed glass sheet having an antilacerative sheet over a surface of the glass sheet facing the interior of the vehicle. The monolithic tempered glass sheets provide minimal sound absorption whereas the double glazed windows provide more sound absorption. The interlayer used in the fabrication of windshields in addition to being effective to secure fragments of glass in place in the event the glass sheet(s) is (are) broken provides the windshield with some sound dampening.
There have been efforts to improve the comfort of passengers in automobiles by reducing transmission of unwanted noise. These efforts have included making laminated glass having interlayer materials with enhanced sound absorbing properties. For example, European Patent Application Number 93104895.3, Publication Number 0 566 890 A1 (hereinafter EPA No. 93104895.3) and European Patent Application Number 95118423.3, Publication Number 0 733 468 A2 (hereinafter EPA No. 95118423.3) discuss automotive sound barrier windshields and panels, respectively.
EPA No. 93104895.3 presents a discussion of sound insulation performance of laminated glass and discloses an interlayer film for use in laminating glass that dampens vibrations. In general, sound insulation performance is measured as the transmission loss (loss factor) at varying frequencies. Sound insulation as discussed in EPA No. 93104895.3 is described in JIS A4708 as a constant value at 500 Hz or above, depending on the sound insulation grade. The sound insulation of glass plates substantially decreases in the frequency range centered around 2000 Hz because of the coincidence effect. The "coincidence effect" stands for the phenomenon wherein, when the sound wave hits the glass plate, the rigidity and the inertia of the glass plate cause propagation of transverse waves on the glass surface, and these transverse waves resonate with the incident sound, resulting in sound transmission.
As discussed in EPA No. 93104895.3 although conventional windshields are superior in terms of the prevention of scattered fragments, they do not avoid the reduction in sound insulating performance caused by the coincidence effect in the frequency range centered around 2000 Hz. On the other hand, based on the loudness-level contour, it is known that human hearing is much more sensitive to sound in the range of 1000-6000 Hz compared with other frequency ranges, indicating that it is important for purposes of sound control to eliminate the drop in sound insulating performance caused by the coincidence effect.
In order to improve the sound insulating performance of laminated glass, it is necessary to mitigate the coincidence effect described above to prevent the decreases in the minimum transmission loss caused by the coincidence effect (hereinafter, this minimum is referred to as the "TL value").
EPA No. 93104895.3 discloses various ways to prevent the decrease in the TL value, such as an increase in the mass of the laminated glass, multi-layered glass, segmentation of the glass area, improvement of the facilities to support the glass plate and improvement in the interlayer film for laminating the glass sheets.
As can be appreciated by those skilled in the art, it would be advantageous to provide articles e.g. glazings to dampen vibrations to reduce transmission noise that are additional products to the products presently available made using the available technology.